This invention relates generally to an electrophotographic printing machine, and more particularly concerns an operator actuatable screen adapted to modulate the light image transmitted to the photosensitive member.
In a typical electrophotographic printing machine, a light image of an original document is projected onto the charged portion of the photoconductive member to record an electrostatic latent image thereon. The light image may be modulated by interposing a screen either spaced from or in contact with the photoconductive member. Screens of this type are known in the graphic arts, as well as in the electrophotographic printing art. The screen improves continuous tone reproductions by resolving the original image into a half-tone pattern of lines or dots. Generally, screens presently employed in an electrophotographic printing machine are spaced from the photoconductive member. The screen customarily includes absorbing rulings which depend on both absorption and diffraction of light to produce the desired light image modulation. Screens having absorbing rulings may be constructed by ruling straight grooves at the desired frequency, typically 24-80 lines/per centimeter, in glass or plastic. These grooves may be filled with an opaque material, and by varying the width of the rulings or modulating the absorption across a ruling, different screens may be produced.
More recently a ruled screen which only phase shifts and diffracts an incoming light wave has been developed. This type of screen is termed a phase screen and includes grooves cut in a nonlight absorbing solid transparent material having a depth on the order of the wave length of light. The grooves are un-filled (contain air). The screen periodcially phase shifts the incoming radiation. One significant advantage of the phase screen is that it does not absorb any light passing therethrough. Consequently, a shorter exposure time with a smaller light source is required when producing a half-tone pattern with a phase screen. A phase screen suitable for use in electrophotographic printing is described in co-pending application Ser. No. 556,387 filed in 1975.
Frequently, in electrophotographic printing, it is desirable to produce a functional copy rather than a pictorial copy of the original document. A functional copy is a copy of a document wherein subtle variations of tone or color are not present, such as in a graph, chart, lines, etc. Functional copying machines generally have great difficulty in forming tone gradations. A pictorial copying machine overcomes this defect by using a screen to produce tonal gradations. In the highlight regions, the halftone pattern may comprise narrow lines or small dots. The lines increase in width or the dots in size from the lighter through the intermediate shades until they merge together in the darker regions.
Numerous patents teach the concept of screening. Exemplary of these patents are U.S. Pat. Nos. 2,598,732; 3,535,036; 3,121,010; 3,493,381; 3,776,633; and 3,809,555. U.S. Pat. No. 3,861,784 teaches an electrically controllable diffraction grating. Of more particular interest is co-pending application Ser. No. 566,873, filed in 1975 which teaches the concept of moving a screen into and out of the optical light path. In this way, the electrophotographic printing machine may function both as a functional or pictorial copier. However, this type of a device requires movement of the screen. It would be extremely desirable to maintain the screen continuously in the optical light path with no movement associated therewith. An optical system employing a screen of this type is described in co-pending application Ser. No. 570,103 filed in 1975. As taught therein, a normally transparent liquid crystal panel is interposed into the optical light path. Energization of the liquid crystal panel forms an opaque screen pattern which modulates the light image passing therethrough.
The foregoing may be achieved by utilizing a selectively actuatable phase screen. Thus, in the functional copying mode the screen is merely a transparent member through which the light image would pass without any phase shift. Contrawise, in the pictorial mode of operation, the screen is actuated to phase shift the light image. In this way, the light image may remain unmodulated, or if appropriate be modulated.
It is well known in the art to employ deformable imaging systems, known generally as "Frost " imaging. The foregoing is described in greater detail in U.S. Pat. Nos. 3,196,012; 3,214,272; and 3,436,216. The techniques of Frost imaging may be applied to make selectable actuatable defraction gratings. For example, co-pending application Ser. No. 580,654 filed in 1975, now U.S. Pat. No. 4,011,009, teaches a variable reflection defraction grating selectively formed by a material deformable in the presence of a conductive grid having an electric field applied thereto, i.e., with a structure similar to the present invention, but employed in a different application.
Accordingly, it is the primary object of the present invention to improve electrophotographic printing machines by employing a selectively deformable operator actuatable screen.